This invention relates to optically resonant sensors with periodic response functions.
Optically resonant structures are used in sensor devices that monitor pressure, temperature, refractive index, gas density, pH and other parameters. Typical sensors are disclosed in U.S. Pat. Nos. 4,678,904 and 4,778,987, which are both entitled Optical Measuring Device Using a Spectral Modulation Sensor Having an Optically Resonant Structure, and which are both assigned to the assignee of the present invention.
For some of these optically resonant sensors, for example pressure sensors and refractive index sensors, the sensor output or response function is periodic or cyclical. That is, the response function values may be the same for different values of the parameter being monitored. This results in sensor inaccuracy, especially at crossover points where the slope of the response function curve goes through zero from positive to negative or vice versa.
At these crossover points, even though the value of the sensed parameter is increasing or decreasing at a relatively constant rate, the change in value of the response function appears to slow down and then reverse itself, indicating that the direction of the change in value of the sensed parameter has also reversed, and thus giving an ambiguous and inaccurate measurement of the sensed parameter.
The range of accurate sensor operation is typically limited to regions of the periodic response function that are monotonic with respect to the parameter being measured. A monotonic function has the property of either never decreasing or never increasing as the independent parameter being measured increases. For example, a sine function is monotonic between any adjacent maximum and minimum
In conventional interferometric sensors, this problem results from the fact that the average gap width between the opposed reflective surfaces is generally several wavelengths deep, relative to the center wavelength of the light source being used to interrogate the sensor. This means that if the external parameter being monitored (e.g., pressure) is applied beyond the full-scale design limit, the sensor output will display a periodic response. This is an inherent property of any interferometric sensor operated at a maximum gap width in excess of approximately 1/4 wavelength. Therefore, at some deflection in excess of approximately 1/4 wavelength, an interferometric sensor will indicate a decreasing value for the parameter when in fact it is increasing. This issue is particularly important for non-fringe-counting interferometric devices with gap width in the undeformed state of less that about 30 .mu.m.
One technique for mitigating this problem is described in the above-referenced U.S. Pat. No. 4,678,904 at col. 9, lines 7-23 and is based on the selection of appropriate sensor design and interrogating light. Another such technique is described at col. 12, lines 1-26 and is based on a ratiometric processing of the sensor output to yield a response function with a period that can approach twice the period of the amplitude of the sensor output light. Even when these techniques are applied, however, the response functions remain periodic the sensors are optimized but continue to give ambiguous readings at both extremes of their monotonic operating ranges.
The periodic nature of the sensor response function is not as critical for some applications as for others. For example, human blood pressure will not exceed about 300 mmHG except under highly unusual conditions. It is then only necessary to design diaphragm stiffness so that the output does not "double back" unless the pressure exceeds, perhaps, 400 mmHg.
However, in some industrial control situations, it is desirable to have a monotonic response for parameter values significantly greater than the full-scale amount.
Therefore, it is an object of this invention to extend the monotonic response range of sensors with periodic response functions.
It is another object of this invention to provide a sensor in which the multiple response cycle phenomena is inhibited, while at the same time providing a sensor design that results in a change in sensor operation when the parameter being measured goes outside the normal operating range of the sensor. In other words, it is an object of this invention to provide an increased monotonic response range and dual-range sensing. For example, in a pressure sensor device, if a transient condition occurs in which system pressure goes to 150% of full-scale and slowly rises, the sensor output according to the present invention indicates a pressure increasing at a different rate rather than an erroneous decreasing pressure.
It is a further object of the invention to increase the tolerance of the sensor beyond its normal operating range to a point that the system is unlikely to erroneously read in-range unless the sensor is subjected to unreasonably high values of the parameter being measured.